I assume you are using the latest version 3.9.
I have it on my site working with php 8.2.
Can you check your handling fee is numeric, if zero, make sure it's not letter o.
if it is numeric try changing line 151 from
Code:
$shipping_cost = ($shipping * $shipping_num_boxes) + (defined('MODULE_SHIPPING_' . $module . '_ZONES_HANDLING_' . $dest_zone) ? constant('MODULE_SHIPPING_' . $module . '_ZONES_HANDLING_' . $dest_zone) : 0) +
(defined('MODULE_SHIPPING_' . $module . '_ZONES_HANDLING') ? constant('MODULE_SHIPPING_' . $module . '_ZONES_HANDLING') : 0);
to
Code:
$shipping_cost = ($shipping * $shipping_num_boxes) + (float)(defined('MODULE_SHIPPING_' . $module . '_ZONES_HANDLING_' . $dest_zone) ? constant('MODULE_SHIPPING_' . $module . '_ZONES_HANDLING_' . $dest_zone) : 0) +
(float)(defined('MODULE_SHIPPING_' . $module . '_ZONES_HANDLING') ? constant('MODULE_SHIPPING_' . $module . '_ZONES_HANDLING') : 0);
If that works do you have strict mode set? Let me know what happens.